~(12)C/~(13)C kinetic isotope effects of the gas-phase reactions of isoprene, methacrolein, and methyl vinyl ketone with OH radicals

摘要

The stable-carbon kinetic isotope effects (KIEs) for the gas-phase reactions of isoprene, methacrolein (MACR), and methyl vinyl ketone (MVK) with OH radicals were studied in a 25 L reaction chamber at (298 ± 2) K and ambient pressure. The time dependence of both the stable-carbon isotope ratios and the concentrations was determined using a gas-chromatography combustion isotope ratio mass spectrometry (GCC-IRMS) system. The volatile organic compounds (VOCs) used in the KIE experiments had natural-abundance isotopic composition thus KIE data obtained from these experiments can be directly applied to atmospheric studies of isoprene chemistry. All ~(12)C/~(13)/C KIE values are reported as e values, where e = (KIE - 1) × 1000‰, and KIE = k_(12)/k_(13). The following average stable-carbon KIEs were obtained: (6.56 ± 0.12)‰, (isoprene), (6.47 ± 0.27)‰ (MACR), and (7.58 ± 0.47)‰ (MVK). The measured KIEs all agree within uncertainty to an inverse molecular mass (MM) dependence of ~(OH)ε(‰) = (487 ± 18)MM~(-1) which was derived from two previous studies [J. Geophys. Res. 2000, 105, 29329-29346;J. Phys. Chem. A 2004, 108,11537-11544]. Upon adding the isoprene, MACR, and MVK ~(OH)ε values from this study, the inverse MM dependence changes only marginally to ~(OH)ε(‰) = (485 ± 14)MM~(-1). The addition of these isoprene ~(OH)ε values to a recently measured set of ~(O3)ε values in an analogous study [Atmos. Environ. 2008,42,8728-8737] allows for estimates of the average change in the ~(12)C/~(13)C ratio due to processing in the troposphere.